This invention relates generally to machines and procedures for incremental printing of text or graphics on printing media such as paper, transparency stock, or other glossy media; and more particularly to alignment provisions in a scanning machine and method that construct text or images from individual ink spots created on a printing medium, in a two-dimensional pixel array. The invention employs threshold printmode techniques to optimize image quality vs. operating time.
The foregoing statement of the field of the invention speaks of xe2x80x9cthresholdxe2x80x9d techniques in the printmode area. This term is used merely to emphasize that the printmode techniques required for practice of the present invention are extremely basicxe2x80x94almost primitive, in comparison with the very sophisticated present state of the printmode or printmasking art.
For example, the related-art section of U.S. Pat. No. 5,677,716 of Cleveland, filed seven years ago, discusses space-rotation, sweep-rotation and autorotation of masks. More-recently filed patent documents introduce manual pseudorandomization, xe2x80x9cneighborhoodxe2x80x9d conditions, balanced randomization and determinism, automatic generation, and real-time generation of printmasksxe2x80x94including columnwise mask generation within each swath, xe2x80x9cprecooked masksxe2x80x9d and xe2x80x9cpop-upxe2x80x9d masks.
Although entirely compatible with all of the advanced techniques just noted, the present invention by comparison actually needs only the most simple or pedestrian tools of the printmode artxe2x80x94little more, in fact, than the concept of a printmask height. Therefore, as will be seen, these techniques are indeed at the threshold of the print-mode arena; yet their use in the present invention confers potent advantages.
(a) The need for registrationxe2x80x94Many incremental printers use more than one array of multiple printing elements, such as for example multipen inkjet printers. Most often the different arrays (xe2x80x9cpensxe2x80x9d or xe2x80x9cprintheadsxe2x80x9d) print in different colors, including black; however, in certain cases some of the different arrays print in different dilutions or saturations of common colors. Other uses of plural arrays may occur.
In such printers it is necessary that markings made by the different arrays be in register with one another. At least markings should be adequately registered to prevent a human viewer from seeingxe2x80x94with the unaided eyexe2x80x94the effects of misregistration.
In general, however, such systems are subject to mutual misalignment of the arrays and therefore misregistration of the markings. Various different kinds of provisions are known for reducing such misalignment to the point at which registration at least satisfies visual requirements.
(b) Tight tolerancesxe2x80x94For convenience and definiteness such provisions can be described with reference to multipen inkjet printers, and more particularly thermal-inkjet units of the Hewlett Packard Companyxe2x80x94such as the models known as PaintJet(copyright), DeskJet(copyright) 1200C and HP(copyright) 2000C. In some of these printers, alignment is achieved mechanically: the pens are aligned to within a dot row, by virtue of individual machining to achieve fine mechanical tolerances.
As a result these printers can print with all their nozzles, and with straightforward good alignment along the direction of printing-medium advance. This is true for the 7 dot/mm (180 dot/inch or xe2x80x9cdpixe2x80x9d) PaintJet printer, and for the 12 dot/mm (300 dpi) model 1200C printer.
It will be understood, however, that this solution to good registration is achieved only at very great cost, since the machining required is time consuming and costly. Furthermore, this solution is commercially feasible only because spacing of the dot rows at {fraction (1/7)} or {fraction (1/12)} mm is relatively coarse in terms of machine-tooling standards.
(c) Reserved elementsxe2x80x94In the HP 2000C, resolution is 24 dot/mm (600 dpi) and at such fine spacings mechanical machining begins to be an uneconomic way to achieve registration. Instead, this printer uses a pen alignment scheme that reserves eight nozzles of each pen for pen-to-pen alignment.
In that system, nominally four nozzles 13K, 15K (FIG. 1) are reserved at each end of the black-ink pen K, and only the nozzles 14K between those two end zones are employed to print. Fixed upper and lower limits 11, 12 are established, reflecting these assumed reservationsxe2x80x94and these fixed limits 11, 12 are applied to the color pens C, M, Y as well as the black pen K.
With drift away from the nominal mechanical alignment, however, for example a particular pen, e.g. the cyan pen C as shown, might be two pixel rows lower than its nominal position, and this would require use of two nozzles from the top nominal end zone. This would leave only two nozzles 13C actually reserved at the top, while in exchange the system would give up two nozzles just above the bottom nominal end zone, leaving unused six nozzles 15C as the bottom end zone. The central group of used nozzles 14C still has the same size in nozzles, but these used elements are shifted upward along the nozzle array by a distance equal to two nozzle spacingsxe2x80x94which most commonly (though not necessarily) is two pixel rows.
In practice any such possibility may occur for any of the pens, including the black pen K or the magenta or yellow pen M, Y rather than the cyan pen C. The particular pattern illustrated, with the magenta pen M shifted upward about six rows and the yellow pen Y at the nominal position, is purely exemplary.
In any event, eight nozzles are always sacrificed to the needs of alignment, and in practice the limit lines 11, 12 are fixed in position relative to the world, or in other words to the pen carriage. The total number of nozzles in each pen is 304; therefore the maximum number of these printing elements that fall within the central regions 15 and can be used is 296.
The remaining {fraction (8/304)} or about 2.7 percent of the nozzle complement is abandoned at the outset. (Of course different reserved and total numbers of nozzles may be present in different models or from different manufacturers; these values are simply exemplary.) From these numbers it can be seen that the end zones 13, 15 of all the pens have been drawn greatly exaggerated relative to the corresponding central regions 14, simply to facilitate clear discussion.
To a person not skilled in this field, such a seemingly small fraction might not appear significant. In this extremely competitive commercial environment, however, these numbers represent a major handicapxe2x80x94for two reasons.
(d) Direct costxe2x80x94First, the provision of eight additional nozzles is far from a small matter. Each nozzle in the nozzle plate is not merely a hole in an amorphous structure, but to the contrary must be accompanied by ink-provision and firing components within the printheadxe2x80x94and these facilities are fashioned in what is the equivalent of a multilayer silicon circuit chip.
The cost of printed-circuits and the like formed in silicon is notoriously expensive, to the extent that common slang in industry refers to silicon xe2x80x9creal estatexe2x80x9d. In addition, the cost of expanding the amount of space used in such a structure is not even linear: the bigger the chip, the more expensive per unit area because of the greater difficulty of working larger chips and the progressively escalating scrap factor.
(e) Hidden costsxe2x80x94The first major handicap of cost does not end there, however, for the silicon real estate carries its operating environment along with it in an ever-widening series of ripple effects. The connecting circuitry must have more contacts, and the print zone must be therefore longer and more expensive.
So must the printer chassisxe2x80x94and accordingly the shipping box. In turn the space and therefore the cost of shipment and inventory storage are implicated as well.
(f) Performance: speedxe2x80x94The second major handicap, even more severe, is reduced throughput. Since the marking-element (e.g. nozzle) array height is in a sense artificially restricted, each swath of marks is likewise restricted in height. A greater number of such swaths is therefore needed to cover any given image height (for instance, a full page) on the printing medium.
Each swath is marked in a single respective pass of the marking-element array (xe2x80x9cpenxe2x80x9d, or printhead) across the sheet of printing medium. The amount of time required to make each pass of the marking-element array across the sheet is essentially independent of the array height.
Therefore the restricted array height can be translated into a greater number of passes per image or page, and a longer time to make those passes. In short, the number of minutes per page increases and the manufacturer""s advertised page-per-minute performance figure falls.
The significance of this is well known to anyone who has ever glanced at a display of competing printers in a retail store. The number of pages printed per minute or hour is one of the two or three most heavily emphasized and most conspicuous characteristics of a desktop computer printer.
While a 2xc2xd% shift alone may not be visible in the published information, naturally several such factors from different causes do become visible and significant. Hence the importance of the eight unused nozzles in terms of performance.
The actual impact of a 2.7% loss of nozzle complement is likely to be extremely nonlinear. For example, in a two-pass printmode if a system comes up just four nozzles short, two additional entire passes must be made to complete the pattern.
At the other extreme, if a system in a two-pass mode happens to reach the end of a pattern and only needs, say, half the nozzles in the printhead for the final swath, then loss or gain of four nozzles might be entirely inconsequential. Since neither of these extreme kinds of circumstances can be predicted generally in advance, it can only be said that the number of available nozzles is very important.
In many practical cases, just a few nozzles more or less can assume a throughput importance out of all proportion to their number. This is surely true in human terms, taking into account the engineering effort regularly devoted even to merely trying to evaluate that importance.
When an overall printhead is just one-half inch tall (including any nozzles that are candidates for reservation) and the image is an integral multiple of a half inch, loss of only two or three nozzles can regularly trigger a requirement for additional passes. This is an adverse case which occurs with particular frequencyxe2x80x94because half-inch nozzle arrays are currently in favor, and many desired images are in integral-inch (e.g., eight by ten) sizes.
(g) Performance: lifexe2x80x94A related phenomenon is the effect on useful life of each printheadxe2x80x94now the number of pages that can be printed before the head wears out. This is particularly important for printheads that are refillable after the ink supply is exhausted.
When the number of swaths per page rises, the number of rows on each page that must be printed by each nozzle also rises. This means that a greater part of each nozzle""s useful life must be expended on each page, and so lowers that life in terms of number of pages.
When several nozzles are worn out, the entire head must be discarded. Hence a 2xc2xd% reduction in nozzle complement translates directly into a 2xc2xd% reduction in pen life.
(h) Statistical wastexe2x80x94The reserved-nozzle approach to alignment is ingenious and extremely useful in comparison with machining to better than {fraction (1/24)} mm. Nevertheless, as has now been shown, that approach is squeezed by two pressures: performance penalties suffered by failing to use all the silicon real estate (and its supporting installed environment) that is actually in place and paid for, and high mechanical cost of adding more capacity to compensate.
If either of these is allowed to control, however, then alignment suffers. This is a particularly acute aggravation because statistically, given the nature of tolerances and departure from nominal conditions, usually the most likely condition is the nominal onexe2x80x94i.e., the used nozzles being the 296 in the middle.
The least likely condition is the extreme case of the used nozzles extending all the way to one or the other end of the array. The intermediate two cases are in general of intermediate likelihood.
Hence the major fraction of the eight-nozzle sacrifice outlined above is made on behalf of a relatively minor fraction of the actually occurring cases of misalignment. To put it another way, the minimum nozzle loss is {fraction (8/304)} or 2.7 percentxe2x80x94regardless of the actual amount of mechanical misalignment.
Every user must pay the full penalties outlined above, even though the printer owned by a representative user has an actual need for perhaps less than half of those penalties.
(i) Conclusionxe2x80x94These registration problems have continued to impede achievement of uniformly excellent incremental printingxe2x80x94at high throughput and very low costxe2x80x94on all industrially important printing media. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.
In preferred embodiments of its first major independent facet or aspect, the invention is a method of printing an image from image data, using plural pens that in general are not perfectly aligned. The method includes the step of determining pen-to-pen mechanical misalignment.
It also includes the step of, based upon the determined misalignment, automatically shifting the data to compensate for at least a portion of the determined misalignment. In addition the method includes the step of automatically printing with the shifted data.
The foregoing may represent a description or definition of the first aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, this facet of the invention obviates the need to sacrifice usable nozzles in return for alignment between pens.
Although the first major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, preferably if the determining step establishes that the pens are aligned within a dot row, then the printing step prints without shifting the data.
Another preference is that the plural pens print respective plural kinds of ink. In this case the plural kinds of ink respectively include plural ink colorsxe2x80x94and, still more preferably, plural ink dilutions. It is also preferable in this case that the invention be for use with a particular printing medium that is substantially insensitive to relative timing of application of the plural kinds of ink respectively.
For such sequence-sensitive inks, the ideal medium in this regard is plain paper. This preference is valid for sequence-sensitive inks generally, but in particular for inks such as typically used in inkjet photographic-quality incremental printersxe2x80x94and most particularly those manufactured by the Hewlett Packard Company.
Also with sequence-sensitive inks one preference is to design the shifting step to compensate for only part of the determined misalignment. In this situation it is also preferable to employ pen-nozzle selections to compensate for at least part of the rest of the misalignment. One common ramification of resorting to such nozzle selections is a small but significant loss of throughput.
On the other hand, it is also possible to configure the shifting step to compensate for all of the determined misalignment. Depending on the degree of sensitivity of the inking system to deposition sequence, this way of operating may be adequate with particular printing mediaxe2x80x94and if so then this operating mode is preferable, as it does not entail any of the throughput loss just mentioned.
In preferred embodiments of its second major independent facet or aspect, the invention is a printer for printing an image from image data. The printer includes plural printheads, each having a multiplicity of marking elements.
Each element is subject to deterioration and shortening of operable life through use, and the plural heads are subject to mechanical misalignment.
The printer also includes some means for extending the life of the marking elementsxe2x80x94and thereby the life of the printheads. For purposes of generality and breadth in discussing the invention, these means will be called simply the xe2x80x9clife-extending meansxe2x80x9d.
The life-extending means accomplish their life-extending effect by distributing use of the marking elements over a maximum number of marking elements. The life-extending means include some means for automatically shifting the data to compensate for at least a portion of the misalignment.
The foregoing may represent a description or definition of the second aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art. In particular, this second aspect of the invention helps counters the problem of limited printhead life, which of course is a concern to all users.
Although the second major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, preferably the life-extending means further include some means for automatically determining the mechanical misalignment.
In this latter case, several valuable alternative preferences obtain. In one of these, the alignment-determining means include alignment data encoded on the pens, and some means for reading the encoded data. In another, the alignment-determining means instead include some means for using the pens to print a test pattern, and some complementary means for reading the test pattern to determine the pen alignment therefrom.
In preferred embodiments of its third major independent facet or aspect, the invention is a printer for printing an image from image data. The printer includes plural pens, each pen having multiple nozzles.
The pens in general are not perfectly aligned. The printer also includes some means for determining pen-to-pen physical misalignment.
Also included are portions of a processor programmed to automatically print the image using substantially all of the multiple nozzles of all of the pens. The processor is programmed to take into account the determined alignment.
The foregoing may represent a description or definition of the third aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular this third aspect of the invention, like the first, in suitable circumstances eliminates the alignment-driven need for discarding a significant part of the nozzle complement. This is a significant advance, because nozzle xe2x80x9creal estatexe2x80x9d xe2x80x94and corresponding capacity on the underlying silicon chip used for firing controlxe2x80x94are limiting factors in the cost of inkjet pens.
Although the third major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, preferably the processor portions are programmed to provide a relative shift of data for the plural pens, to compensate for imperfect alignment of the pens.
In this case ordinarily the physical misalignment includes relative displacement of the pens toward lower or higher positions. A corresponding preference is that the data shift include raising data for a lowest pen or lowering data for a highest pen, or both.
Another preference is that the determining means include alignment data encoded on the pens, and some means for reading the encoded data. In this case the determining means preferably further include carriage datum-point alignment calibration data ascertained at manufacture of the apparatus, and portions of a processor programmed to automatically take into account the carriage datum-point alignment calibration data.
Yet another preference is that the determining means include some means for using the pens to print a test pattern. This preference also includes provision of corresponding means for reading the test pattern, to determine the pen alignment from the read pattern.
Alternative preferences in this regard include using the pens to eject drops, under automatic processor control, and a drop detector to determine their location. For example a shutter and optical sensor may preferably serve as such a detector.
All of the foregoing operational principles and advantages of the present invention will be more fully appreciated upon consideration of the following detailed description, with reference to the appended drawings, of which: